Regulation of gene expression during development occurs at multiple levels as indicated by the recent identification of numerous cis-regulatory sequences, transcription complexes, microRNAs (miRNAs), and additional classes of non-coding RNAs. miRNAs are small, single-stranded RNAs that play important roles in proliferation, differentiation, tumorigenesis, and apoptosis via their ability to silence target genes. miRNAs function primarily as repressors of gene expression, typically acting at the level of translational regulation of gene expression. miRNAs can also promote mRNA degradation and, in quiescent cells, activate translation. The ability of miRNAs to regulate large sets of target genes may provide “developmental switches” during lineage differentiation of various cell types including stem cells.